1. Technical Field
The present invention relates to a descaling nozzle for removing scale from a surface of a rolled steel manufactured by hot rolling and a cemented carbide nozzle tip which is useful for this nozzle.
2. Description of the Related Art
A hot rolled steel is manufactured by heating a steel slab to about 1100 to 1400° C. in a heating furnace under an oxidizing atmosphere and hot rolling the heated slab by a rolling mill. Due to the heating in the above-mentioned heating furnace, scale comprising iron oxide forms on the surface of the steel slab, and if hot rolling is performed without removing this scale, scale cracks are formed on the surface of the rolled steel and lower the product value. Descaling nozzles have been proposed for removing such scale by a high-pressure jet of water.
Japanese Patent Application Laid-Open No. 24937/1996 (JP-8-24937A) discloses a steel plate surface cleaning method in which the surface temperature of a steel plate is heated to not lower than 850° C. and liquid droplets, generated in the liquid drip flow region of a discharged liquid flow from a nozzle, collide with the surface of the steel plate for cleaning. This literature also discloses that a liquid discharged from a nozzle is collided with the surface of a steel plate containing Si in an amount of not less than 0.5 weight %.
Japanese Patent Application Laid-Open No. 334335/2000 (JP-2000-334335A) discloses a high-pressure jet nozzle comprising an elliptical opening which forms the entrance of an exit flow path, and a supply flow path which narrows towards the elliptical opening, in which only the side wall of the exit flow path in the direction of the major axis of the ellipse enlarges in the direction of flow, and the side wall in the direction of the minor axis of the ellipse extends substantially parallel to the axial line of the supply flow path.
However, according to these nozzles, water should be jetted with a high pressure and it is difficult to remove scale efficiently with a low pressure or a low flow rate.
Japanese Patent Application Laid-Open No. 263124/2000 (JP-2000-263124A) discloses a descaling nozzle, for removing scale by discharging water from a nozzle at a discharge pressure of not lower than 40 MPa and for colliding the water onto the surface of a steel plate with a distance between a discharge orifice and the steel plate of not longer than 150 mm, in which the discharge direction of the discharge flow spreads in the width direction within a plane perpendicular to the central axis of the nozzle, and the discharge flow has an erosion thickness angle in the range of 1.5 to 2.5° in the thickness direction perpendicular to the width direction. This literature also discloses a flat spray nozzle for descaling, wherein an enlarged passage is provided at the upstream side of the discharge orifice, and the inner diameter of the enlarged passage is 7 to 10 times that of the discharge orifice and the length of the enlarged passage is not less than 100 mm. Further, the document discloses a method of descaling a steel plate surface in the hot rolling process of a high-Si-containing steel, in which water is discharged from the nozzle at a discharge pressure of not less than 40 MPa with maintaining a distance from the discharge orifice to the steel plate of 75 to 150 mm.
However, with the above-described descaling nozzle and descaling method, it is required to discharge water at a high pressure and a high flow rate in order to make the erosion amount large. Furthermore, since the inner diameter of the enlarged passage is large with respect to the discharge orifice, the nozzle size becomes large.
Japanese Patent Publication No. 73697/1994 (JP-6-73697B) discloses a scale removal nozzle comprising a rectifying flow path in which a rectifier is disposed therein and is substantially equal in diameter across the entire length, a constricted flow path formed at the downstream side of the rectifying flow path and becomes gradually smaller in diameter towards the downstream side, and a jetting passage formed at the downstream side of the constricted flow path and extends to a jetting opening which is opened at the bottom of a groove formed at the front end face of the nozzle.
Japanese Patent Application Laid-Open No. 94486/1997 (JP-9-94486A) discloses a descaling nozzle comprising a flow path which becomes gradually smaller in diameter towards the downstream side, and a slit-like orifice communicating with the flow path and extending to a front end, the flow path and the orifice being formed in a main nozzle body made of a cemented carbide. This nozzle has a concave surface which is formed at the front end of the main nozzle body and has an inclined side wall that narrows towards the upstream side, and a jetting opening which is opened at the bottom of the concave surface and extends to the orifice. This literature discloses that the concave surface may have a circumferential wall extending in the axial direction from the upstream end of the inclined wall.
The nozzles described in these literatures are useful for improving the wear resistance of the orifice due to ultrahigh-pressure water. However, it is necessary to discharge water with a high pressure and a high flow rate in order to realize a high descaling efficiency.
DE No. 92U17671 Specification illustrates a nozzle comprising a discharge orifice formed at the front end of the nozzle, a first conical flow path spreading at an angle of about 50° towards the upstream side from the discharge orifice, a first cylindrical flow path extending in the upstream direction from the upstream end of the first conical flow path and having an inner diameter of about twice the inner diameter of the discharge orifice, a second conical flow path spreading at an angle of about 70 to 80° in the upstream direction from the upstream end of the first cylindrical flow path, a second cylindrical flow path extending in the upstream direction from the upstream end of the second conical flow path and having an inner diameter of about four times the inner diameter of the discharge orifice, and an inclined flow path spreading gradually and extending in the upstream direction from the upstream end of this cylindrical flow path (FIG. 1 in DE No. 92U17671 Specification).
However, even with this nozzle, water should be discharged at a high pressure and a high flow rate in order to realize a high descaling efficiency. Further, since two conical flow paths are formed, the nozzle has a complicated structure essentially. Furthermore, it is especially difficult to prepare a nozzle tip having two conical flow paths from cemented carbide.
Thus an object of this invention is to provide a descaling nozzle and a cemented carbide nozzle tip that realize efficient scale removal even at a low pressure and/or a low flow rate.
Another object of this invention is to provide a descaling nozzle and a cemented carbide nozzle tip that improve the descaling performance (or efficiency) with inhibiting the cooling of the steel plate.
It is still another object of this invention to provide a descaling nozzle and a cemented carbide nozzle tip that are compact and high in descaling performance (or efficiency).
It is a further object of this invention to provide a descaling nozzle and a cemented carbide nozzle tip useful for descaling of steel material in hot rolling.